The present invention relates to a photoscreening camera system. In particular the present invention relates to a method and an apparatus for detecting the photorefractive error of a patient's eye for diagnosing amblyopia and for detecting misalignment of the eyes (strabismus) and opacities in ocular media and other observable ocular abnormalities.
The principles behind photorefraction is covered in an article entitled "Quantitative Photorefraction Using An Off-Center Flash" Physiologic Optics, 1989, Vol. 65, pp. 962-971. The underlying principal of photoscreening is that by taking a special photographic picture of a child's eye, the factors which can lead to amblyopia (or lazy eye) can be detected. These three main factors are unequal refractive errors, misalignment of the eyes, and opacities in the ocular media such as cataracts. One can also detect asymmetry of the lids, asymmetry of the pupils, and any external abnormalities around the eye in the photograph.
The prior art literature on eccentric flash photoscreening systems describes either one- or two-flash systems. A two-flash system is described in U.S. Pat. No. 4,523,820 to Kaakinen. The Kaakinen patent covers only two flashes used simultaneously or one flash with two objectives. His patent does not cover one-flash systems, even when one flash is used to check one meridian and then that one flash is rotated to another meridian. U.S. Pat. No. 4,669,836 to Richardson covers a one-flash camera system using an LED for FIXATION control as well as a very elaborate structure to hold the child's head rigid. The test distance of this system is 2.4 meters and it utilizes the long distance from the subject to the camera to achieve measured sensitivity. The eccentricity of the flash with respect to the objective of the camera is large, with the flash being placed outside of the objective of the camera. The flash does not rotate and one can only measure one meridian of abnormalities. The camera system is therefore insensitive to the detection of astigmatism and small degrees of refractive error, and it is not fully portable nor hand-held. It also does not utilize instant film for ease and rapidity of interpretation.
U.S. Pat. No. 4,586,796 to Molteno utilizes a ring flash which is placed very close to the edge of the camera objective. A full ring flash is not disclosed in the Kaakinen patent. The same optical principles apply in all eccentric photorefraction, but note that the eccentricity in the Molteno device is larger because its flash is significantly farther away from the center of the objective or the edge of the lens aperture compared to present invention and therefore the sensitivity of that camera system is less.
The description of the optics of the photorefraction are described by Dr. Howard Howland in an article entitled "Optics of Photoretinoscopy: Results from Ray Tracing", American Journal of Optometry and Physiologic Optics, Vol. 62, pp. 621-625, 1985. The article describes isotropic photorefraction with the light flash right in the center of the lens objective. It discloses a flash unit which is placed centrally in the optical axis of the system and creates an eccentric aperture by creating a shield below and around the optical flash. This system does not have a central lens aperture (a slit) nor use an eccentric flash.
However, by utilizing the formula (disclosed in the article) for determining the optimal sensitivity of the camera and dioptric thresholds, one can obtain a sensitivity, based upon an eccentricity of the flash edge from the edge of the lens aperture of about 1 mm, which is significantly better than even the best system (which was previously described as the Molteno system), with a 0.09 diopter sensitivity with an 5 mm pupil.
The article "Eccentric Photorefraction: Optical Analysis and Empirical Measures" by Bobier and Braddick, American Journal of Optometry, and Physiologic Optics, Vol. 62, pp. 614-620, 1985, describes an analysis of the optics of eccentric photorefraction. The article describes eccentricity as the distance above the extreme edge of the camera lens and describes the eccentricity of the flash source beyond the margin of the lens aperture. The theoretical curves of best sensitivity which are obtained are different from the theoretical curves of the present invention due to the present invention's minimal eccentricity of the flash from the edge of the aperture lens which permits a null interval of nearly zero (0.0250). Using this formula, the minimal defocus of the eye which one can detect is 0.10 diopters with a 10 mm pupil, which agrees with the findings and the sensitivity of the camera of the present invention. The minimal eccentricity is obtained by using ASA 3000 black and white film or other high sensitivity imaging films and a narrow aperture and flash eccentricity. Also by proper rotation of the flash source, one can have the effective output of the flash at the edge of the strobe, further increasing the sensitivity of the camera system.
Accordingly, it would be desirable to create a slit aperture with a minimal eccentricity approximately 1 mm by placing the strobe edge as the edge of the lens aperture while using a high ASA film or a light sensitive video camera detection system or other imaging device.
It is desirable to use a high speed (black and white) film e.g. (ASA 3000) to create a small aperture and eccentricity, since with using a slower ASA e.g. (ASA 600) color film, the minimum aperture would be 5-6 mm (rather than the 3 mm aperture which can be utilized with black and white film) to have enough light enter the aperture of the camera for proper exposure.
It is also desirable to provide for a photoscreening camera for detecting misalignment of the eyes by evaluating the corneal reflection caused by the appearance of the flash on the eye similar to the Hirschberg test used in clinical practice and by detecting an intensity difference of the red reflex, which is seen as a brighter image in a misaligned eye, so described as the BRUCHNER reflex. Thus, a brightness difference of the red reflex of the eyes can indicate misalignment of the eye even when the corneal reflection appears to be symmetrical. Brightness differences can show misalignment of greater than 2.degree.-3.degree..